Not sure about the GPU, it does seem a bit on the warm side, but it is a fairly hot part and that is within operating temps. However, that's definitely on the hot side for the HDD, so if anything I'd up that fan speed a little bit.

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I have the new 27" iMac and when checking iStat my GPU Diode temperature seems very high. It's at 187 degrees fahrenheit, sometimes when the computer is doing nothing at all.

I have the AMD Radeon 6970M - 2GB VRAM, if that helps.

Is this too high? Should I install SMC fan control?

I have the same config, and that seems warmer than it should be. I don't think you need to install SMC fan control though - it looks like your optical drive fan has already spun up to compensate -- this is similar to what I see on my machine after I've been gaming for a few hours & the room is hot, though my GPU is more ~170deg.

If the GPU really should be idle, I'd say its a potential problem. try the PRAM reset suggested (or a cold boot), let the machine idle for 20 minutes, and see what the temperature reads... I would expect numbers closer to your CPU temp...

I have the same config, and that seems warmer than it should be. I don't think you need to install SMC fan control though - it looks like your optical drive fan has already spun up to compensate -- this is similar to what I see on my machine after I've been gaming for a few hours & the room is hot, though my GPU is more ~170deg.

If the GPU really should be idle, I'd say its a potential problem. try the PRAM reset suggested (or a cold boot), let the machine idle for 20 minutes, and see what the temperature reads... I would expect numbers closer to your CPU temp...

Thanks for all the suggestions. It is very warm where I live, it was over 93 degrees Fahrenheit outside when I took the screenshot. That may be one of the problems.

I noticed the same thing a few days ago on my iMac (27" i7, 2011). My GPU temp was about 175° F with nothing running.

After doing a few experiments, here is what I observed:
My ambient room temp is about 77° F.

I have a second display hooked up through the Mini-display port (20" Apple Cinema Display (ADC) via an Apple DVI to ADC Converter. When the display is plugged in, the GPU temp idles at about 170° F. When I unhook the second monitor, the temp drops to about 145° F within 4 minutes.

I used smcFanControl to bump the Optical drive fan speed to a constant 1400rpm and the GPU temp stays at a comfortable 145° F. The GPU heatsink is above the optical drive so the air flows over it on its way out the top.